This invention relates to selected data compression for digital pictorial information and, in particular, the supply of digital information to digital display screens. This includes a variety of forms of digital display screen including large screen LED displays, LCD displays, LCD projectors, plasma television and similar forms of apparatus.
Original video and television technology utilized analogue signals for the recording, transmission and driving of displays.
The cathode ray tubes (CRTs) still predominate the worldwide market for display apparatus in a form of televisions or similar. Therefore, it will be sometime before digital displays predominant to the extent that original data captured in the form of video, film or other analogue recording mechanisms are entirely replaced by digital mechanisms facilitating direct digital recording to digital transmission and reproduction.
The operation of a CRT is such that the brightness or intensity of a particular pixel on a screen is determined by the quantity of electrons accelerated by the cathode ray tube onto that point of the screen. However, the power or voltage supplied to the cathode ray tube leads to a non-linear response in the number of electrons accelerated.
This lack of linearity is well known and referred to as the gamma effect.
Different transmission or reproduction standards set different gamma functions to account for this lack of linearity. The gamma function not only varies between industry standards such as NTSC or PAL but also with proprietary brands of monitors and similar display technologies.
Current recording technologies can involve analogue cameras which themselves work on a non-linear basis and although they may need to be compensated to the particular gamma function of the display on which the signal is reproduced, there is little identifiable loss of definition during this compensation process.
If information is recorded in a digital form, various intensities are recorded by discreet binary numbers. For example, if the information is recorded using 8-bit technology, different levels of intensity are graduated according to the 256 possible binary numbers available in an 8-bit binary number. This is substantially a linear representation of the intensity. Therefore, to allow such digital information to be displayed utilizing conventional non-linear technology, it is necessary to add a gamma function to this information to provide a non-linear correlation between the intensity and the individual steps from each bit of information.
Regardless of the method of recording the data, as presented for reproduction, the data is likely to come in analogue or digital form having a gamma function. It is only if the recording process has been performed directly on digital equipment and no gamma function has been added as it is directly intended for use on a digital display that such a function may not exist. Transmissions by broadcast also benefit from a lack of linearity in reducing noise.
The result of this is that most digital displays need to work with such non-linear data.
When an analogue signal is received to represent pictorial information for a digital display, one of the first steps is to convert the analogue signal into digital information for processing. Whether converted from analogue or initially digital, the signal then needs to be linearized to remove the gamma function and provide the data in a manner that the digital display, that operates substantially linearly, will correctly represent the intended image. If the processed signal is supplied directly to the digital display, apparent visual distortions can occur in the low intensity colours. For low intensity colours, a single step in the binary information, once the gamma function has been removed, can lead to a distinct visual change in intensity. This leads to an effect referred to as mach banding where regions of low intensity colour approaching black can show distinct bands of colour where a single binary step in the digital information needs to represent a large percentage change in colour from the original non-linear signal. The reverse is true at high intensity colours so that in any high signal intensity colour that has the gamma function removed can easily be represented on a digital display. Large percentage errors between the anti-gamma digital representation and the non-linear signal occur at low intensity colours with minimal errors at the higher intensity colours.
To overcome this effect, different approaches have been taken in the past.
One approach is to use some error diffusion in the representation of the digital display. In effect, in an area where a band boundary would normally be apparent, some pixels in the lower intensity band are provided with the value of the higher intensity band and vice versa to provide the appearance of graduation in the intensities rather than any distinct step.
Although this effect can work for some digital displays, other digital displays utilize significantly larger and more apparent pixels. This may particularly be the case for large indoor or outdoor LED display screens where each individual pixel is a substantially larger unit and can be visually apparent on its own. Also, the total number of pixels in the display may be less in some displays making each individual pixel more important to the overall image. The use of error diffusion on such displays leads to a loss of definition rather than simply overcoming the banding.
The alternative course of action that is taken with most LED displays and indeed with other digital displays is to increase the accuracy of the digital information subsequent to removal of the gamma function by using an increased number of bits.
Typically, if an 8-bit digital signal is used as a direct representation of the nonlinear signal, an anti-gamma function needs to be applied to linearize the non-linear data. The application of this function loses some accuracy as the smaller binary numbers lose definition. For example, if an 8-bit signal provides a representation of the numeral 16 representing a small value of the 256 possible graduations, the application of the anti-gamma function may indicate that the true value should be, perhaps, 0.65. Such a number cannot be represented by a subsequent 8-bit binary number. Hence the output may simply be xe2x80x9c1xe2x80x9d. Due to the nature of the anti-gamma calculation, the number 25 may normally equal 1.45 but still need to be represented as xe2x80x9c1xe2x80x9d in the recalculated linearized 8-bit data. This occurs only on those lower intensity values. Generally, the data will be truncated or rounded to lose significant digits.
Greater accuracy can be employed by outputting the 8-bit data, once the anti-gamma function has been applied as a 10-bit binary number to allow a greater degree of accuracy on these low intensity colours.
Of course, some systems utilize higher bits from the outset although such apparatus is naturally more expensive as greater processing abilities are needed throughout the entire system. The use of 8-bit technology has become a standard for the more economic forms of digital pictorial information such as a standard video.
The loss of definition in colours once the anti-gamma function is applied means that the original 8-bit information comprising some 256 discreet levels is reduced upon output. For an anti-gamma function of 2.2 which is substantially the standard for NTSC signals, only approximately 184 discreet output colours are possible using an 8-bit output. However, if the 8-bit input receives the anti-gamma function and is provided as a 10-bit output, approximately 233 colours are possible. With higher bit outputs, even more colours are available with a 16-bit output providing substantially the same 256 discreet colours which are considered sufficient to substantially correspond to an original analogue signal.
The difficulty in providing an increase in data bits once the anti-gamma function has been applied is that all the downstream equipment must similarly be able to transmit or process 10-bit data.
Again, looking specifically at the case of large screen LED displays, such displays are normally provided as a series of interconnectible display panels to allow easy shipment, assembly and standard control of the overall screen. The provision of 10-bit data paths to the control board on each discreet interconnectible segment of the screen would be relatively expensive. Therefore, the prior art solution is to provide the anti-gamma function and the increase to 10-bit data only once the data has reached each of the control boards provided on each interconnected segment of the screen. This decreases the need for expensive data paths to each segment of screen although increases the cost of the control boards as each control board must include suitable apparatus to perform this function. Generally this is facilitated through an Eprom provided on the board having an 8-bit input and a 10-bit output. The costs of supply connection, and programming of such Eproms on each individual board add significantly to the overall cost of the display screen.
It is an object of the present invention to provide selected data compression for digital pictorial information to overcome some of these problems of the prior art or at least provide the public with a useful choice.
Accordingly, in the first aspect, the invention may broadly be said to consist in a method to transmit or store image data for display on a digital display system comprising:
applying an anti-gamma function to linearize an incoming data value and providing said linearized value as a higher order number;
receiving said linearized higher order number and, if below at least a first threshold value capable of being transmitted or stored on a lower order data system, transmitting or storing said linearized higher order number on said data system for use with a digital display system; and
compressing said higher order numbers above said at least first threshold to also fit said lower order data system for storage or transmission and subsequent uncompression and use with said digital display system.
Preferably said at least first threshold value is set to a value less than or equal to a value transmittable or storable on a data system of nxe2x88x921 channels where n equals the maximum order of the lower order data system.
Preferably higher order values above said first threshold are compressed to values transmittable or storable on a data system of nxe2x88x921 channels where n equals the maximum order of the lower order data system.
Preferably said first threshold value is set to a value corresponding to a value represented by a single active bit in a binary sequence being a value of 2x where x is a whole number.
Preferably x equals nxe2x88x921.
Preferably said uncompressed and said compressed values are transmitted or stored on nxe2x88x921 channels and the remaining channel is used to indicate the state of compression of the value.
Alternatively, m channels or bits are used as indicating bits and multiple compression states are utilized.
Accordingly, in a second aspect, the invention may broadly be said to consist in an apparatus to transmit or store image data for use with a digital display system comprising:
conversion means to apply an anti-gamma function and linearize a data value to a linearized higher order number;
selection means to select linearized higher order numbers below at least a first threshold value;
compression means to compress linearized higher order numbers above said at least first threshold value; and
output means connectable to a lower order communication or storage system to transmit or store said selected and said compressed values.
Preferably said conversion, selection and compression is performed by a programmable memory apparatus.
Preferably said programmable memory apparatus comprises an Eprom.
Alternatively, said conversion, selection and compression may be performed, at least in part, by a logic circuit.
Preferably said selection selects according to at least a first threshold value less than or equal to the maximum value transmittable or storable on nxe2x88x921 channels where n equals the order of said lower order communication or storage system.
Preferably said at least one remaining channels is used to indicate the compression state of the value.
Accordingly, in a third aspect, the invention may broadly be said to consist in an apparatus for the uncompression of transmitted or stored linearized data from a lower order data system into a higher order data system for use with a digital display where said linearized data comprises uncompressed values below at least a first threshold and compressed values above said at least first threshold and an indicating bit or bits to indicate the state of compression of the value comprising:
switching means driven by said indicating bit or bits to select between at least two separate logic operations for incoming linearized data values;
a direct output of said incoming value if said switching means indicates an uncompressed value has been received; and
decoding bits of said lower order incoming values to higher order bits of said higher order data system if said indicating bit or bits indicate a compressed value.
Accordingly, in a fourth aspect, the invention may broadly be said to consist in a method for the uncompression of transmitted or stored linearized data from a lower order data system into a higher order data system for use with a digital display where said linearized data comprises uncompressed values below at least a first threshold and compressed values above said at least first threshold and an indicating bit or bits to indicate the state of compression of the value comprising:
utilizing said indicating portion to switch the output between at least two alternative inputs;
a direct output of the incoming value if the indicating portion indicates an uncompressed value; and
decoding incoming data bits to higher order output data bits if said indicating bit or bits indicates a compressed value.